Elastomeric materials are traditionally produced through extrusion processes to make sheets or strands of elastomeric materials. Subsequent cutting of the elastomeric sheets or stands to the desired size and/or shape and joining the cut pieces to a substrate are typically required. Overall, the above processes involve multiple steps to produce the finished product and may generate much wasted materials. In view of these drawbacks, the ability to print or spray elastomeric compositions is particularly advantageous. The printing and spraying processes may deliver the elastomeric materials directly onto the substrate, thus, avoid the drawback of a multi-step process. These processes may also deliver the elastomeric materials only to targeted areas where elastic properties are needed, thus, minimize the amount of waste generated. Moreover, these processes may also provide controlled delivery of varying amounts of elastomeric materials to discrete areas in a single step, which is difficult, if not impossible to achieve by traditional extrusion/molding processes.
Fiber spinning, melt blowing, and other processes require low melt viscosity materials, which are typically thermoplastic polymers such as polyethylene, polypropylene, polyesters, and polyamides. Elastomeric materials having suitably low melt viscosity for these processes are olefinic elastomers made from single site catalysts. Many elastomeric materials, such as styrenic block copolymer compositions, are generally considered not suitable for such low viscosity processes. The discoveries by the present inventors have made the elastomeric compositions of the present invention uniquely suitable for these low melt viscosity processes.
It is well known that plasticizers, viscosity modifiers and processing oils may be used to lower the viscosity and improve the melt processability of TPE's or mixtures. However, due to their low molecular weight and their softness and/or fluidity down to room temperature, these agents tend to reduce the mechanical properties of the TPE's and blends. In contrast, the phase change solvents are solid-like at or below body temperature, thus, they may function like reinforcing particles (i.e., fillers) in the TPE's and blends. Moreover, the phase change solvents, due to their chemical formula and molecular weights, may be intimately mixed the TPE's and function like compatibilizers. When they solidify, they may be fairly homogeneously dispersed throughout the TPE matrix. Homogeneous distribution of reinforcing particles is desirable since few stress concentration spots (detrimental to mechanical properties) are created in such structures. Their compatibilizing function may also lead to reduced phase sizes and reduced stress concentrations at the interfaces between the phases of the TPE's.
Additionally, oils are added to reduce cost and further increase the process characteristics of polymeric materials. However, the addition of highly branched and low molecular weight oils is deleterious to mechanical properties.
Block copolymers comprising one or more alkenylarene polymer block and one or more olefinic polymer block are generally known as thermoplastic elastomers (TPE's). The block copolymers are elastomeric in the sense that they typically have a three-dimensional, entangled (alternatively known as “physically crosslinked”) structure below the glass transition temperature (Tg) of the styrenic block such that they exhibit elastic memories in response to external forces. The block copolymers are thermoplastic in the sense that they can be softened or melted above the glass or crystalline transition temperature of the alkenylarene block, processed, and cooled/solidified several times with little or no change in physical properties (assuming a minimum of oxidative degradation).
These block copolymers are known to have high strength and elasticity at ambient temperatures. The high strength and elasticity of these block copolymers are due to the microphase separated network structure wherein the olefinic blocks and the alkenylarene blocks separate from structurally dissimilar blocks and entangle with structurally similar blocks to form separate domains. The olefinic blocks typically have a glass transition temperature below ambient temperature, thus, they are relatively free to move about and form the soft, rubbery phase at or above ambient temperature. In contrast, the alkenylarene blocks have a glass and/or crystalline transition temperature above ambient temperature, thus, they are relatively immobilized in the entangled state and form the hard phase. However, at body temperature, the copolymers may begin to lose their mechanical properties after some time. The deterioration of properties appears to be associated with the copolymer movements, especially the movements of the alkenylarene blocks. At body temperature, sometimes accompanied with tension or load, the previously immobile alkenylarene blocks begin to slip pass neighboring alkenylarene blocks. Since the alkenylarene blocks form the hard phases, which are primarily responsible for the mechanical properties, such motions of the alkenylarene blocks adversely affect the mechanical properties of the copolymer. The relative hardness of the alkenylarene blocks can be assessed by the value of a measured glass transition temperature. A high glass transition temperature indicates strong interactions and restricted motion of the styrenic chains, which ultimately leads to tensile strength. Conversely, low glass transition temperatures indicate the motion of molecules can occur easier and causes a drop in tensile performance.
Plasticizers or processing oils are often added to the block copolymers to lower the viscosity and improve the processability of the block copolymers. Other polymers may also be added to compatibilize the blends and/or improve the mechanical properties. Blends comprising block copolymers are described in U.S. Pat. Nos. 3,562,356 (Nyberg et al.); 4,704,110 (Raykovitz et al.); 4,578,302 (Schmidt et al.); 5,503,919 (Litchholt, et al.); 5,540,983 (Maris et al.); 6,117,176 (Chen); and 6,187,425 (Bell et al.).
However, the addition of the plasticizers and/or some processing oils lower the strength and elastic properties of the block copolymer compositions. Such oils often contain a high degree of branching, low molecular weights, and polar functionalities. The degree of branching and low molecular weights contributes to reduced glass transition temperatures and changing the relaxation times of the hard phase within the thermoplastic elastomer. The alteration of the glass transition temperature and molecular relaxation times are well-known to affect viscoelastic properties.
Therefore, it is desirable to provide a novel combination of materials that reduces the viscosity and improves the processability of block copolymer compositions without substantially compromising their mechanical properties.
It is also desirable to provide a material that exhibits a phase change as the temperature is raised and/or lowered such that the novel material effects very sharp changes in the characteristics (e.g., viscosity) of the block copolymer compositions at or around the phase change temperature of the material.
Moreover, it is desirable that the viscosity of the phase change solvent and of its block copolymer blends can be varied over a broad range to achieve the suitable viscosity for different fabricating processes, such as extrusion, injection molding, melt spinning, blow molding, spraying, printing, coating, and the like.